An apparatus obtains a plurality of waveforms representing measurements of a physical characteristic of a machine's operation and a plurality of performance results of the machine corresponding respectively to the plurality of waveforms, each of the performance results indicative of the machine's performance under conditions at which the measurement represented by the corresponding waveform was made. The apparatus divides each of the waveforms into a common plurality of intervals and calculates, for each of at least one of the intervals, an influence value that represents a degree of influence of the plurality of waveforms on the plurality of performance results over the interval.